The invention is based on a priority application EP 02360121.4 which is hereby incorporated by reference.
The invention relates to an optical switch fabric comprising at least one micro mirror array with tilting micro mirrors for individually switching optical paths for optical signals between input and output ports, especially between fibres or waveguides gathered in at least one fibre array.
Optical switches, for so-called optical cross connects or optical patch panels, based on a free space three dimensional micro mechanical mirror architecture work by tilting said micro mirrors to establish optical paths between input ports to selected output ports. In a so-called z-type architecture, an array of input fibers or waveguides each with a collimator lens (often called collimated fibres) is aligned such, that each of the optical input signals falls onto a corresponding micro mirror element of a first set of mirrors, arranged in a first mirror array, that can direct the incident light rays by tilting the mirrors around two axes. A second array of collimated output fibers or waveguides is aligned to a second set of mirrors, arranged in a second mirror array. By precisely tilting the corresponding pair of mirrors, the light ray of an arbitrary input fibre can be directed to a selected output fibre establishing a corresponding optical path.
An alternative architecture of an optical switch using micro mirrors comprises only one array of mixed collimated input and output fibres, one set of micro mirrors and a fixed auxiliary mirror. The light ray from an arbitrary input fibre is directed to a corresponding micro mirror. Said micro mirror is tilted such, that it reflects the received light via the auxiliary mirror to a selected further micro mirror, that is tilted to reflect said light back to the corresponding output fibre.
To achieve and maintain high coupling efficiency between input and output fibres, regardless of changes in the environmental conditions, e.g. temperature change of the mirror chip or ageing, the optical connections, i.e. the optical paths between input fibres, mirrors and output fibres, should remain extremely stable over a long period of time. Thus, the angular positions (or angles) of the mirrors must be controlled to maintain optimal optical connections.
One prior art solution for a control of mirror positions concerns a method of tapping signal light from each output fibre for supervision of the actual transfer behavior of the optical switching unit or switch fabric. However, this solution results in a very bulky and costly arrangement, as to tap the light of each output fibre, for each of said output fibre a corresponding power splitter and an individual detector must be provided. Furthermore, this method does not allow any precise control of the focal points or spots of the light signals at the corresponding fibre interfaces or micro mirror array.
An alternative prior art solution for a control of mirror positions concerns the use of capacitance sensors for determining the actual angular positions of the mirrors. However, this method does not give any feedback of the position of said focal points; therefore deviations from the optical path can not be observed and corrected.
The object of the invention is to disclose an optical switch fabric and a corresponding control method to precisely control the optical elements of the switch fabric for maintaining high coupling efficiency between input and output fibres (or waveguides) regardless of environmental influences and without any need for tapping light from an output fibre.
The basic idea of the invention is to control the tilting mirrors of a micro mirror array of an optical switch fabric for individually switching optical signals between input and output ports, e.g. consisting of fibre ends gathered either in a combined fibre array or in separated fibre arrays, such, that the position of the signal light spots onto the optical targets of said micro mirrors, either further micro mirrors or collimating lenses of corresponding output fibres, are observed by realising an exact optical image onto an observation or detector array. Said optical image is realised by a beam splitter adjusted to partly reflect the optical signals coming from micro mirrors to be controlled. A control signal is generated to correct the angular positions of said micro mirrors to couple exactly the corresponding input and output ports. To achieve a correct optical alignment of the optical unites, i.e. mirrors, beam splitters and fibre array(s), an optical control signal is transmitted from an input fibre to an output fibre, each the intensity of the reflected parts of said control signal by the beam splitters and the intensity of the remaining part not reflected by the beam splitters is detected, and second feedback signals out from said detected intensities are generated each to control the positions of the mirrors and the fibre array(s) to each other.